1. Field of the Invention
The present invention relates to a radiation imaging apparatus. Specifically, the present invention relates to a radiation imaging device which is capable of obtaining past radiation images and present radiation images of a single subject, suitable for extracting differences between the two by inter image calculations, such as a so-called temporal subtraction process.
2. Description of the Related Art
There are known methods of detecting temporal changes in subjects, based on temporal subtraction images. The temporal subtraction images are obtained by performing inter image calculating processes, such as a subtraction process, on a radiation image of a subject which has been obtained in the past (hereinafter, referred to simply as “past image”) and a radiation image of the same subject which is obtained presently (hereinafter, referred to simply as “present image”). The temporal subtraction image represents differences between the past image and the present image. In the field of medicine, for example, a method for detecting tumors that develop over time employing temporal subtraction images is known. This method obtains temporal subtraction images from past images and present images of the same subject, and detects the presence of new tumors based on the temporal subtraction images.
In cases in which temporal subtraction is performed, artifacts are generated unless the portions of the past images and present images that correspond to the subject are not completely matched (excluding the portions that have changed over time). The artifacts become barriers to the detection of temporal changes, therefore it is preferable that they are removed as much as possible.
Accordingly, methods for eliminating artifacts have been proposed, for example, in U.S. Patent Application Publication No. 20010048757 and U.S. Pat. No. 6,751,341. In these methods, image processes, such as affine transform, are administered on one or both of the past and present images, to match the positions of the images that correspond to the subject.
Factors that cause artifact generation include differences in the position of a subject. In the case that the subject is a living organism, the respiratory phase and the cardiac phase (the phase of the constriction/expansion movement of the heart) of the subject are also factors that contribute to artifact generation. For example, in the case that the subject is a human body, if the subject's position is different, positional misalignment as a whole, centered on the bone portions, is likely to occur. If the respiratory phase of the subject is different, the degree of inspiration differs, therefore the size of the lungs and the positions of the organs in the periphery thereof change, causing positional misalignment of the organs. Further, if the cardiac phase of the subject is different, the size of the heart and the arrangement of blood vessels in the periphery thereof differs, causing positional misalignment of the blood vessels.
In the positioning methods by the aforementioned image processes, rough positioning is possible, yet fine positioning of detailed elements is difficult, and there is a limit in the artifact eliminating capabilities thereof. Accordingly, it is difficult to detect fine elements, such as tumors having diameters of about 5 mm, which are indicative of the early stages of lung cancer.
A method can be considered, in which imaging conditions of a present image are aggressively controlled such that the arrangement of structural elements of the subject approximate that of a past image, thereby enabling the generation of temporal subtraction images having few artifacts. However, such a method has not yet been proposed.